Apparatus for distributing melt in a multi-level stack mold

ABSTRACT

An improved melt distribution system and method is provided for a multi-level stack mold having three or more moving platens. The injection machine communicates with a bifurcated sprue bar for providing pressurized melt through the first moving platen to a central distribution manifold in the second moving platen. From the central distribution manifold, the flow of pressurized melt is distributed to the first and third platens for transfer to a plurality of mold cavities.

FIELD OF THE INVENTION

This invention relates to injection molding and, in particular, to thedistribution of melt through a multi-level stack mold.

BACKGROUND OF THE INVENTION

Stack molding advantageously permits molding machine output to bemultiplied without appreciably increasing the, overall size of themachine. However, stack molding has the disadvantage that a moreextensive melt runner system is required to extend through the movingplatens to reach the cavities.

It is well known that the configuration of a melt distribution paththrough an injection stack mold critically affects the overall partquality. Failure to provide a melt flow under equal pressure to eachmold cavity will result in differential filling of the cavities and willnot produce consistent parts from cavity to cavity. Typically, evenpressures from cavity to cavity are ensured by providing equal lengthrunner passages with an identical number of bends of identical radiusand arc. This is usually achieved by locating the main distributionmanifold centrally within the stack mold, usually within one of themoving platens.

To transfer pressurized melt to the moving platen (ie. across the firstparting line between the stationary platen and the moving platen),typical applications have provided a sprue bar extending through thestationary platen from the machine nozzle across to the moving platen,as shown in U.S. Pat. No. 5,011,646 to Berteschi. This structure has thedisadvantages that the sprue bar is in the way when the mold is open anddamages falling parts. Furthermore, the sprue bar interferes with anyrobotic arm which may be provided to assist with part ejection, moldface preparation or the like.

The extensive runner system makes the use of a sprue bar system evenmore unsatisfactory in multi-level stack molds. For example, U.S. Pat.No. 5,370,523 to Kushnir and European Patent Application No. EP-911139disclose a centrally located sprue bar arrangement for feedingpressurized melt to the various moving platens of a multi-level stackmold. The presence of the central sprue bar, however, limits the abilityof mold larger parts, due to the interference of the sprue bar locationand the mold cavity placement.

U.S. Pat. No. 5,846,472 to Rozema et al. teaches a more complexeccentric sprue bar arrangement for use in three- and four-level stackmolds. The numerous sprue bars, however, only compound the problemsnoted above. Furthermore, the presence of multiple sprue bars can limitthe size of parts that can be molded.

Another problem associated with multi-level stack molds is that themaximum height of parts to be molded is limited by the distance that themolding machine can move between its open and closed positions and theamount of space required for each mold level. For example, thetelescoping configuration of the sprue bars of EP-911139 must be mademore extensive if wider platen separation is desired. Rozema et al.teach providing a bifurcated sprue bar to permit greater separation ofplatens upon mold parting, however, the limitations of Rozema et al.have been noted above.

Accordingly, there is a need for a melt distribution arrangement formulti-level stack molds which has generally equal length melt paths foreach mold level. Furthermore, there is a need for a melt distributionarrangement for a multi-level stack mold which does not require acentrally-located sprue bar, thereby allowing single parts to be moldedwhich extend across the central mold axis. There is yet a further needfor a melt distribution arrangement for a multi-level stack mold whichutilizes a minimal number of sprue bars to minimize interference withthe molding process. There is also a need for an improved drool controlapparatus for use in multi-level stack molds.

SUMMARY OF THE INVENTION

In one aspect the present invention provides a multi-level stack moldcomprising a stationary platen, a first, second and third movingplatens, the moving platens being moveable between an “open” and“closed” position in a longitudinal direction generally parallel to agenerally centrally disposed mold axis, a plurality of mold cavitiesdefined between the stationary and moving platens, a first mold cavitymanifold disposed in the first moving platen in communication with atleast one of said mold cavities, a second mold cavity manifold disposedin the third moving platen in communication with at least one of saidmold cavities and a sprue bar assembly extending through the firstplaten for selectively providing a flow of pressurized melt from thestationary platen to the second platen for distribution to the first andsecond mold cavity manifolds.

In a second aspect, the present invention provides a multi-level stackmold comprising a stationary platen, a first, second and third movingplatens, the moving platens being moveable between an “open” and“closed” position in a longitudinal direction generally parallel to agenerally centrally disposed mold axis, a plurality of mold cavitiesdefined between the stationary and moving platens, a first mold cavitymanifold disposed in the first moving platen in communication with atleast one of said mold cavities, a second mold cavity manifold disposedin the third moving platen in communication with at least one of saidmold cavities, a bifurcated sprue bar assembly .extending through thefirst platen for selectively providing a flow of pressurized melt fromthe stationary platen to the second platen, the sprue bar assemblyhaving a first portion and a second portion in flow communication, thesecond portion separably matable with the first portion, the sprue barassembly being disposed eccentrically from the central mold axis and adistribution manifold disposed in the second platen in flowcommunication with the sprue bar assembly for selectively providing theflow of pressurized melt to the first and second mold cavity manifolds,whereby when the mold is in its closed position, the first and secondportions of the sprue bar assembly are in communication with each otherand the distribution manifold is in communication with the first andsecond mold cavity manifolds.

In a third aspect, the present invention provides a multi-level stackmold, the stack mold comprising a stationary platen, a first, second andthird moving platens, the moving platens being moveable between an“open” and “closed” position in a longitudinal direction generallyparallel to a generally centrally disposed mold axis, a plurality ofmold cavities defined-between the stationary and moving platens, a firstmold cavity manifold disposed in the first moving platen incommunication with at least one of said mold cavities, a second moldcavity manifold disposed in the third moving platen in communicationwith at least one of said mold cavities, a distribution manifold in oneof said moving platens and a bifurcated sprue bar assembly connected to,and in communication with, the distribution manifold for providing aflow of pressurized melt to the distribution manifold, the sprue barassembly being disposed eccentrically from the central mold axis,whereby the sprue bar assembly and the distribution manifold arearranged to be non-coaxial.

In a fourth aspect, the present invention provides a method ofdistributing pressurized to a melt in a multi-level stack mold having astationary platen, a first, second and third moving platens, the movingplatens being moveable between an “open” and “closed” position in alongitudinal direction generally parallel to a generally centrallydisposed mold axis, a plurality of mold cavities defined between thestationary and moving platens, a first mold cavity manifold disposed inthe first moving platen in communication with at least one of said moldcavities, and a second mold cavity manifold disposed in the third movingplaten in communication with at least one of said mold cavities, themethod comprising the steps of transferring the pressurized melt fromthe stationary platen to a distribution manifold in the second movingplaten and distributing the pressurized melt to the first and thirdmoving platens via the first and second mold cavity manifolds fordelivery to a plurality of mold cavities.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, and to show moreclearly how it may be carried into effect, reference will now be made byway of example to the accompanying drawings.

The drawings show the preferred embodiments of the present invention, inwhich:

FIG. 1 is a sectional view of a four-level stack mold apparatusaccording to a first embodiment of the invention, shown in the closedposition;

FIG. 2 is a sectional view of the apparatus of FIG. 1, shown in the openposition;

FIG. 3 is a sectional view of a three-level stack mold apparatusaccording to a second embodiment of the invention, shown in the closedposition;

FIG. 4 is a sectional view of the apparatus of FIG. 3, shown in the openposition;

FIG. 5 is a sectional of view of a melt control valve for use in thepresent invention, shown in a open position,

FIG. 6 is a sectional view of a melt control valve of FIG. 5, shown in aclosed position,

FIG. 7 is a not-to-scale sectional view of a drool control apparatus foruse in the present invention, shown in a first position;

FIG. 8 is a not-to-scale sectional view of the drool control apparatusof FIG. 7, shown in a second position;

FIG. 9 is a not-to-scale sectional view of the drool control apparatusof FIG. 7, shown in the third position;

FIG. 10 is a not-to-scale sectional view of the drool control apparatusof FIG. 7, shown in a fourth position;

FIG. 11 is a not-to-scale sectional view of the drool control apparatusof FIG. 7, shown in a fifth position;

FIG. 12 is a sectional view of the drool control apparatus of FIG. 8,taken along the line 12—12.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1, a stack mold embodying an improved meltdistribution system in accordance with the present invention is showngenerally at 10.

Stack mold 10 comprises a stationary platen or back plate 12, a firstmoving platen 14, a second moving platen 16, a third moving platen 18and a fourth moving platen 20. Platens 12, 14, 16, 18 and 20 areselectively matable at a first parting line 22, a second parting line24, a third parting line 26 and a fourth parting line 28, respectively.Stack mold 10 has a mold injection axis 30 defining longitudinal openingand closing directions for the moving platens.

An injection molding machine (not shown) has an injection nozzle 32which communicates with a heated runner system 34 via a sprue bushing36. Heated runner system 34 comprises a back plate runner passage 38, asprue bar assembly 40, a central distribution manifold 42, a firstplaten manifold 44 and a third platen manifold 46. First platen manifold44 and third platen manifold 46 communicate with a plurality of moldcavities (not shown), defined between the platens at the parting lines,via a plurality of mold cavity gates 48.

Sprue bar assembly 40 comprises a first portion 40′ and a second portion40″ selectively joined by a first melt flow control valve assembly 49.Inside sprue bar assembly 40, a first runner passage 50 communicateswith a second runner passage 60, via control valve 49. First flowcontrol valve assembly 49 comprises a first runner gate 52, selectivelycloseable by a first valve pin 54 actuated by a first actuator 56, and asecond runner gate 62, selectively closeable by a second valve pin 64actuated by a second actuator 66. First flow control valve assembly 49is preferably of the construction more particularly described in U.S.Pat. No. 4,212,626 to Gellert, and further described below.

Central distribution manifold 42 communicates with first platen manifold44 via a second flow control valve assembly 69. Second flow controlvalve assembly 69 is preferably constructed identically to first flowcontrol valve assembly 49, and comprises a third runner passage 70,having a third runner gate 72, a third valve pin 74 and a third actuator76, communicating with a fourth runner passage 80, having a fourthrunner gate 82, a fourth valve pin 84 and a fourth actuator 86.

Central distribution manifold 42 also communicates with third platenmanifold via a third flow control valve assembly 89. Third flow controlvalve assembly 89 is preferably constructed identically to the first andsecond flow control valve assemblies, and comprises a fifth runnerpassage 90, having a fifth runner gate 92, a fifth valve pin 94 and afifth actuator 96, communicating with a sixth runner passage 100, havinga sixth runner gate 102, a sixth valve pin 104 and a sixth actuator 106.

Sprue bar assembly 40 is disposed substantially parallel to injectionaxis 30, along a sprue bar axis 110. Sprue bar assembly 40 passesthrough first moving platen 14 via a first platen through-pass 112 (seeFIG. 5). First platen through-pass 112 permits sprue bar assembly 40 todeliver pressurized melt directly to central distribution manifold 42disposed in second platen 16. Central distribution manifold 42communicates at an angle (preferably 90°, although almost any angle lessthan 180° is possible) with sprue bar 40 to transfer pressurized melt toa central portion of second platen 16. Melt is transferred to firstplaten manifold 44 and third platen manifold 46 for delivery to the moldcavities, as described above. The angle between distribution manifold 42and sprue bar assembly 40 is required to permit actuator 66 to avoidinterference with the flow of pressurized melt in runner system 34.Likewise, angle connections are required between the various portions ofrunner system 34 at each actuator location, (ie actuators 56, 66, 76,86, 96 and 106).

As stated, the flow control valve assemblies are preferably designed inaccordance with U.S. Pat. No. 4,212,626. Referring to FIGS. 5 and 6,first flow control valve assembly 49 preferably comprises first runnergate 52 sealable by a tip 58 of first valve pin 54. In its “open”position (FIG. 5), first valve pin 54 is withdrawn from gate 52, byfirst actuator 56 (see FIG. 1), to permit a flow of pressurized melt toexit first runner passage 50 via gate 52. Second valve pin 64 operatesin a similar fashion, and cooperates with first valve pin 54 to allowthe flow of pressurized melt to enter gate 62 when tip 68 is withdrawntherefrom. Referring to FIG. 6, prior to (or contemporaneously with) theparting of the mold, pins 54 and 64 are moved by their respectiveactuators such that tips 58 and 68 seal gates 52 and 62, respectively.With the runner gates sealed in this manner, the platens of the mold maythen be parted (as shown in FIG. 2) without fear of melt drooling fromgates 52 or 62.

Thus, first flow control valve assembly 49 has “open” (FIG. 5) and“closed” (FIGS. 6) positions. As will be understood by one skilled inthe art, the actuation of the valve pins is timed and synchronized suchthat the flow control valve assembly is “open” when the platens of themold are closed, and the valve pins of the control valve assembly aremoved to their “closed” position upon, or prior to, the opening of stackmold 10.

The reference marker “P/L” in the Figures represents the nominal partingline upon which the flow control valve assembly is parted. For firstflow control valve assembly 49, it will be understood, with reference toFIGS. 1 and 2, that control valve assembly 49 does not part along one ofthe mold parting lines 22, 24, 26 or 28, but rather its own individual“parting line” within first platen 14.

Second flow control valve assembly 69 and third flow control valveassembly 89 are preferably constructed and operated in a manner similarto as first flow control assembly 49. Second and third flow controlvalve assemblies will have a parting line (“P/L”) which coincides withparting lines 24 and 26, respectively.

The flow control valve assemblies may also optionally provide a cavityanti-drool means shown at 170, 170′ and 170″, as will be described inmore detail below.

When stack mold 10 is closed, the flow control valve assemblies are intheir respective “open” positions, as described above. The moldingmachine may then be actuated to force a flow of pressurized melt vianozzle 32 into back plate runner passage 38. The pressurized melt istransferred, via heated runner system 34, to the plurality of moldcavities in stack mold 10. After the injection phase and packing phase,as is known in the art, the valve pins of the flow control valve unitsare actuated by their respective actuators to close the flow controlvalve units. Stack mold 10 may then be opened, as shown in FIG. 2, toeject the molded parts from stack mold 10. Upon opening of mold 10, thebifurcated sprue bar assembly 40 separates into its first and sectionportions 40′ and 40″, which are withdrawn from first platen through-pass112 as the mold opens. Once the mold is open, the molded parts may beejected from their respective cavities. The mold may then be closed, andthe flow control valve assemblies opened in preparation for the nextmolding cycle.

First platen through-pass 112 advantageously permits sprue bar assembly40 to directly communicate with central distribution manifold 42 insecond platen 16. This configuration permits the more centraldistribution of pressurized melt to the first and third platen manifold,thereby facilitating a more balanced runner length design throughout therunner system. It will be understood, however, that through-pass 112strictly need not be provided, but rather sprue bar 40 may pass aroundfirst platen 14 instead.

Referring to FIGS. 3 and 4, the bifurcated sprue bar assembly designaccording to the present invention may be equally applied to othermulti-level stack mold configurations, such as a three-level stack mold10′. Three-level stack mold 10′ has platens 12, 14 16 and 18, in asimilar configuration as described above. Distribution manifold 42communicates with first and third manifolds 44 and 46, respectively, asdescribed above, although manifold 44 now has halves 44 and 44′, tomatch the modified configuration of the 3-level mold, as will beunderstood by one skilled in the art.

Referring to FIGS. 7-12, a cavity anti-drool mechanism for use with themelt distribution system of the present invention will now be described.Note that, as will be apparent to one skilled in the art, FIGS. 7-11 arenot shown on the same scale as FIGS. 1-4. In particular, the length ofsecond runner passage 60, between, by-pass 174 and distribution manifold42 has been shortened for convenience of illustration.

FIGS. 7-11 show the cavity anti-drool system combined with a flowcontrol valve system of the type described above with reference to FIGS.5 and 6. It will be apparent to one skilled in the art that theanti-drool mechanism described herein need not be limited to suchcombination, but may also be used alone, or in conjunction with anotherflow control valve configuration.

Referring to FIG. 7, positioned within first platen 14 is drool controlassembly 170 which comprises a piston 172 and a by-pass chamber 174,being an enlarged section of second runner passage 60. Piston 172 isintegrally incorporated in the second valve pin 64 and positioned on thestem of valve 64 such that piston 172 is positionable, in a firstposition, in a restricted section 176 of second runner passage 60 and,in a second position, in by-pass chamber 174.

For reasons which will become apparent below, piston 172, restrictedsection 176 and by-pass chamber 174 are shaped and sized tosubstantially block second runner passage 60 in its first position butpermit melt flow therearound when piston 172 is in its second positionin by-pass chamber 174, as described below.

The operation of drool control assembly 170 is synchronized with moldinjection as will now be described. Referring to FIG. 8, in preparationof the molding phase, actuator 66 moves second valve pin 64 to its“open” position, as shown in FIG. 8. In this position, piston 172 ispositioned in by-pass chamber 174. At the same time, tip 68 of secondvalve pin 64 withdrawn from second gate 62 and tip 58 of first valve pin54 withdrawn from first gate 52 to permit flow through flow controlvalve assembly 49, although as discussed above, these flow control valveassembly components do not necessarily form part of the anti-droolcontrol apparatus.

When positioned as shown in FIG. 8, melt is permitted to flow from themolding machine through first runner passage 50 and into second runnerpassage 60, around piston 172 through by-pass chamber 174 and intodistribution manifold 42, for delivery to the mold cavities. Once themold cavities are filled, the molding pressure is maintained to apply apacking pressure, as is known in the art.

Referring to FIG. 9, upon completion of the packing phase, actuator 66moves second valve pin 64 and piston 172 “upstream” (ie. away, fluidlyspeaking, from the mold cavities), thereby causing piston 172 to enterrestricted section 176. Upon the movement of piston 172 into restrictedsection 176, the melt material in second melt passage 60 on the upstreamside of piston 172 is forced back into restricted section 176, alongsecond melt passage 60 in the upstream direction.

Referring to FIG. 10, as piston 172 travels upstream through restrictedsection 176, a pressure drop is created in the melt material immediatelybehind (ie “downstream” from) piston 172, which pressure drop iscorrespondingly transmitted to distribution manifold 42 and, ultimately,to gates 72 and 92. (Simultaneously, as second valve pin 64 moves to itsfull-stroke or “closed” position, the flow control valve assembly 49partially closes by tip 68 of second valve pin 64 seating in gate 62 toclose the downstream half of flow control valve assembly.) It will beunderstood that the stroke length of piston 172 is chosen to obtain thedesired pressure drop in the runner system of first moving platen 14 toachieve the intended anti-drool performance.

Referring to FIG. 11, once second valve pin 64 is in its “closed”position, the first valve pin 54 is closed, with tip 58 fully seated ingate 52. Flow control valve assembly 49 is now fully closed. Mold 10 maynow be opened, along first parting line 22, (see FIG. 2 or 4) to permitthe molded parts to be ejected from mold 10. By means of the pressuredrop imparted by the drool control assembly 170, the decompressed meltin distribution manifold 42 advantageously reduces the tendency of themelt to drool from the gates 72 and 92.

Once the molded parts have been ejected from the mold, the mold may beclosed and the molding machine readied for the next molding cycle.

Referring to FIG. 12, in the preferred embodiment, piston 172 has asubstantially identical, but slightly smaller, cross-section to secondmelt passage 60, but also has a longitudinal cutout 180 through itsthickness. Cutout 180 permits some melt to flow past piston 172 as it isstroked upstream during its decompression cycle, thereby reducing theresistance pressure the upstream melt exerts on piston 172. Thus, cutout180 advantageously allows the size of piston 62 to be reduced. The sizeand shape of cutout 180 can be tuned to a particular molding applicationto optimize decompression performance in the stack mold manifold, aswill be apparent to one skilled in the art.

The construction of anti-drool assembly 170, as described above, ispreferably substantially the same as is used for anti-drool controlassemblies, 170′ and 170″. Anti-drool assemblies 170′ and 170″ may beused advantageously in fourth and sixth runner passages 80 and 100 toinhibit drool at gates 48 in thermally gated molding applications.

The term “piston” as described in reference to body 172 need not be apiston in the conventional sense, but may be any body capable of movingmelt upstream in the runner system to effect a decompression downstreamof the body.

Although it is desirable to actuate drool control assembly 170 prior toparting the mold, so that the melt material displaced upstream of piston172 by the actuation of assembly 170 may return into first runnerpassage 22, it will also be understood that drool control assembly 170may also be configured to actuate contemporaneously with the parting ofthe mold, provided that a suitable bleed arrangement, as will beunderstood by one skilled in the art, is made for the upstream meltdisplaced by the stroke of piston 172 in second runner passage 60.

While the above description constitutes the preferred embodiment, itwill be appreciated that the present invention is susceptible tomodification and change without parting from the fair meaning of theproper scope of the accompanying claims.

We claim:
 1. A multi-level stack molding apparatus comprising: (a) amold having a stationary platen and at least a first, second and thirdmoving platens defining a plurality of mold cavities therebetween, saidfirst moving platen disposed between said stationary platen and saidsecond moving platen, said mold moveable longitudinally between an openand a closed position; (b) a first runner manifold in said first movingplaten, said first runner manifold in communication with at least one ofsaid plurality of mold cavities; (c) a second runner manifold in saidthird moving platen, said second runner manifold in communication withat least one of said plurality of mold cavities; and (d) a distributionmanifold disposed in said second moving platen and adapted tocommunicate with said first and second runner manifolds when said moldis closed, (e) a sprue bar assembly having a first sprue bar portionmounted to said stationary platen and a second sprue bar portion mountedto said second moving platen, said first sprue bar portion communicatingwith a molding machine nozzle and extending to a first closeable runnergate, said second sprue bar portion extending from a second closeablerunner gate and communicating with said distribution manifold, saidfirst and second runner gates being in communication when said mold isclosed, said closeable runner gates being openable when said mold isclosed to permit a flow of pressurized melt from said injection machinenozzle to be provided through said sprue bar assembly, distributionmanifold and said runner manifolds to said plurality of mold cavities.2. The multi-level stack molding apparatus of claim 1 wherein saidcloseable runner gates are closed with a valve pin.
 3. A multi-levelstack molding apparatus comprising: (a) a mold having a stationaryplaten and at least a first, second and third moving platens defining aplurality of mold cavities therebetween, said first moving platendisposed between said stationary platen and said second moving platen,said mold being moveable longitudinally between an open and a closedposition; (b) a first runner passage mounted to said stationary platen,said first runner passage extending between a molding machine nozzle anda first closeable runner gate; (c) a second runner passage mounted tosaid second moving platen, said second runner passage extending from asecond closeable runner gate and communicating with said plurality ofmold cavities, said second runner passage communicating with a portionof said plurality of mold cavities through a third closeable runnergate, said first and second closeable runner gates communicating witheach other when said mold is closed; and (d) a third runner passagemounted to said second moving platen, said third runner passageextending from a fourth closeable runner gate and communicating withsaid portion of said plurality of mold cavities, said third and fourthcloseable runner gates being capable of communicating with each otherwhen said mold is closed, said closeable runner gates being openablewhen said mold is closed to permit a flow of pressurized melt to beprovided from said injection machine nozzle to through said runnerpassages to said portion of said plurality of mold cavities.
 4. Themulti-level stack molding apparatus of claim 3, wherein said closeablerunner gates are closed with a valve pin.
 5. The multi-level stackmolding apparatus of claim 3 wherein said first moving platen is adaptedto permit said first and second closeable runner gates to communicatetherethrough when said mold is closed.
 6. The multi-level stack moldingapparatus of claim 3 wherein said second runner passage communicateswith a second portion of said plurality of mold cavities through a fifthcloseable runner gate, and the molding apparatus further comprises (f) afourth runner passage mounted to said third moving platen, said fourthrunner passage extending from a sixth closeable runner gate andcommunicating with said second portion of said plurality of moldcavities, said fifth and sixth closeable runner gates being capable ofcommunicating with each other when said mold is closed, said runnergates being openable when said mold is closed to permit a flow ofpressurized melt to be provided from said injection machine nozzle tothrough said first, second, and fourth runner passages to said secondportion of said plurality of mold cavities.
 7. The multi-level stackmolding apparatus of claim 3 further comprising a first valve unitadapted to selectively close said first runner gate, and a second valveunit adapted to selectively close said second runner gate.
 8. Themulti-level stack molding apparatus of claim 7 further comprising athird valve unit adapted to selectively close said third runner gate,and a fourth valve unit adapted to selectively close said fourth runnergate.
 9. The multi-level stack molding apparatus of claim 7 wherein atleast one of said valve units includes a drool control member mountedthereto and positionable in the runner passage.
 10. The multi-levelstack molding apparatus of claim 9 wherein said first valve unit isdisposed in said first runner passage and said second valve unit isdisposed in said second runner passage, and wherein said first andsecond valve units each include a drool control member mounted theretoand positionable in said respective runner passage.
 11. The multi-levelstack molding apparatus of claim 9 wherein said drool control membercomprises a body, and wherein when said valve unit is in an openposition said body is in a bypass position where pressurized melt insaid runner passage is permitted to flow past said body, and whereinwhen said valve unit is in a closed position pressurized melt in saidrunner passage is at least partially impeded from flowing past saidbody.